1. Technical Field
This invention relates to marine cargo transfer devices and, more particularly, to a motion compensation apparatus used in conjunction with a hoist or marine crane for compensating for the relative movement caused by wave motion when cargo is transferred between a vessel and a stationary body such as an offshore drilling platform or dock.
2. General Background
The relative movement caused by wave motion between a vessel and a stationary body such as an offshore drilling platform or dock can create hazardous conditions during loading and unloading operations. This is especially true in less than ideal weather conditions. Unless some compensation is made for the relative movement between the two bodies the risk of bodily injury to crew members and damage to the crane, vessels or cargo is often times unacceptably high. My earlier U.S. Pat. No. 4,310,277 discloses a device useful in this regard, over which the present invention is an improvement.
A device for transferring heavy loads at sea is disclosed in U.S. Pat. No. 3,945,508 where a device for connecting two ships employs an inverted V-shaped cargo boom having one leg mounted on each ship. With such arrangement, however, transfers can only occur during periods of calm seas.
U.S. Pat. Nos. 3,428,194 and 4,027,800 are both directed to marine gantry cranes for handling cargo aboard ship while U.S. Pat. No. 3,757,678 is directed to a marine crane for handling logs. None of these patents deals with connecting two bodies to compensate for relative movement caused by wave motion.
A combined marine ramp transfer and mooring system is illustrated and described in U.S. Pat. No. 4,003,473. The ramp connects the two vessels so that personnel and/or material can travel between the two.
Trolley-type high-line transfer systems between ships are taught in U.S. Pat. Nos. 3,012,518 and 3,787,031 where a high-line cable is connected between ships so that cargo can be transferred.
Motion compensation systems for cranes and other cargo transfer equipment mounted on a vessel have been developed. In U.S. Pat. No. 4,021,019 a heave compensated crane is disclosed for holding a load at a fixed position and in U.S. Pat. No. 3,916,811 a tide compensation system for a vessel is disclosed.
Marine crane motion compensation systems are the subject of U.S. Pat. Nos. 3,591,022; 3,662,991; and 4,126,298 where under rough sea conditions a line connected to the cargo carrying surface of the other vessel in addition to a load carrying line ensures non-impact and precise placement of the cargo. Such a system can be supplemented with means to ensure that the cargo is lifted from the vessel at the wave crest of the relative motion cycle as disclosed in U.S. Pat. No. 4,025,055. Another motion compensator is taught in U.S. Pat. No. 4,174,188 where a supplemental cargo support is mounted on the deck of a vessel and is raised or lowered at an appropriate time for compensating for wave motion.
3. General Discussion of the Present Invention
An apparatus is provided in accordance with the present invention where cargo can be safely transferred between two bodies which are movable relative to each other such as an offshore drilling platform or dock and a vessel, or between a drilling ship and a vessel.
A linkage is connected between the two bodies which accommodates any relative movement between them. The linkage is made up of three arms, a first arm pivotally connected to one of the bodies to accommodate pivotal movement in a horizontal plane, a second being pivotally connected to the first arm at one end and to a third arm at the other end with relative pivotal movement through a vertical plane being accommodated at these latter two pivot points. The outer end of the third arm is connected to the other body through a vertically disposed frame with movable connections between the arm and frame and the frame and vessel which accommodate motion between the body and the linkage including pitching/heaving of the second body, rotation of the second body, and lateral movement of the second body which in the preferred embodiment is compensated to maintain a position of the second body generally proximate and under the outer end of the third arm.
While it is to be understood that the apparatus is applicable to two moving bodies, it is best described and illustrated using it between a stationary body, such as an offshore drilling platform, and a moving body, such as a supply vessel.
A marine crane or hoist on the stationary body has a hoist line which can be moved into and out of engagement with a guide pulley connected to the linkage. A trolley in the form of a movable pulley connected to an endless line, a screw-thread or a cogged-wheel apparatus extending between two stationary rotatable pulleys or drive gears which are mounted on the third arm engages the hoist line on the side opposite where it engages the guide pulley. Movement of the trolley back and forth along the third arm in cooperation with either the reeling in or playing out of the hoist line respectively, depending on whether the hoist line is moving away from or toward the vessel, will operate to move the hoist line and transfer cargo connected to the hoist line along the third arm.
The linkage provides a physical connection between the two bodies and operates as a stable guide for the cargo by compensating for relative movement between them. Utilization of a guide pulley and the trolley in the form of a movable pulley for engaging the hoist line provides a path for the cargo to a point where it can either be lifted vertically away from the influence of the moving vessel or deposited onto the deck of the vessel. Connection of the hoist line to the linkage through the pulleys where the linkage operates as an extension of the deck of the moving vessel allows cargo to be lifted from or deposited on the deck with a minimum of relative movement between the hoist line and deck.
In this way cargo is maintained under control both during the lifting and depositing phases of the operation as well as during transfer from the vessel to the stationary body and vice-versa even under less than ideal weather conditions. There is little danger of the connection between the two bodies breaking since the pivot points of the linkage and its connections to the two bodies compensate for all motions imparted by wave to the vessel. The cargo can be safely transferred between the deck of the vessel and a point where it is beyond the influence of wave motion.
One alternate embodiment shows the apparatus of the present invention with a multi-line/traveling block arrangement for heavy lifts.
In another embodiment, the lift line portion is transported laterally from the platform toward the vessel by a laterally moving sheave assembly carried longitudinally by the third arm.
Another embodiment can be used for transporting lighter loads, mooring lines, personnel baskets and the like between platform and vessel. That embodiment features a lifting sheave movable laterally upon and supported by the third arm.
Another embodiment features a spring loaded lowering device.
Another embodiment shows the apparatus of the present invention featuring a multi-line traveling block.